This application claims the benefit of Korean Patent Application No. 2002-1381, filed Jan. 10, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a developing apparatus of an electrophotographic printer, and more particularly, to a developing apparatus having a structure of attaching/detaching a developing roller to/from a photoreceptive drum.
2. Description of the Related Art
In general, an electrophotographic printer, such as a color laser printer, includes an image forming apparatus which forms an electrostatic latent image on a photoreceptor and develops the electrostatic latent image using toner. The developed image is transferred to a predetermined transfer medium and retransferred to a sheet of paper therefrom. The transferred image of the paper is heated and pressed to be completely fixed on the paper.
FIG. 1 shows a conventional electrophotographic printer. Referring to FIG. 1, the conventional electrophotographic printer includes a photoreceptive drum 100 which is a photoreceptor, a charger 111 charging the photoreceptive drum 100, an LSU (laser scanning unit) 112, as an exposure unit, scanning a light beam onto the charged photoreceptive drum 100 to form an electrostatic latent image, a developing unit 113 developing the electrostatic latent image by using toner of four colors: yellow (Y), magenta (M), cyan (C), and black (K), a transfer belt 114 sequentially receiving the four colored images developed on the photoreceptive drum 100 to overlap one another, a first transfer roller 114a transferring the developed images formed on the photoreceptive drum 100 to the transfer belt 114, a second transfer roller 114b transferring the image of four overlapping colors on the transfer belt 114 to a sheet of paper, and a fusing unit 115 fusing the transferred image by heating and pressing the paper. The conventional electrophotographic printer further includes a paper cassette 116 containing sheets of paper, a cleaning unit 117 cleaning a surface of the photoreceptive drum 100, a discharger 118, and a transfer path 119 along which the paper is exhausted.
In the above structure, the image of four overlapping colors is formed as follows. First, when the photoreceptive drum 100 is charged by the charger 111 to a predetermined electric potential, the LSU 112 scans the light beam to form an electrostatic latent image of a first color to be developed with first color toner. A developing unit of the first color first accesses toward the surface of the photoreceptive drum 100 to develop the electrostatic latent image of the first color. For example, if a yellow color is to be developed first as shown in FIG. 2A, a yellow color developing unit 113-Y accesses the photoreceptive drum 100 to develop the electrostatic latent image formed on the photoreceptive drum 100 using yellow color toner. Here, remaining developing units of other colors maintain a particular gap with the photoreceptive drum 100 to be separated from the surface of the photoreceptive drum 100. The developed yellow image is transferred to the transfer belt 114. When the development of the yellow color is complete, the yellow color developing unit 113-Y retreats from the photoreceptive drum 100.
Next, the electric potential supplied to the photoreceptive drum 100 is removed from the photoreceptive drum 100 by the discharger 118. Then, another electrostatic latent image of a second color is formed on the photoreceptive drum 100 by charging and exposing of the photoreceptive drum 100. For example, if the second color to be developed is magenta as shown in FIG. 2B, a magenta color developing unit 113-M accesses the photoreceptive drum 100 to develop the electrostatic latent image using magenta color toner. The developed magenta image is transferred to the transfer belt 114 to which the yellow image has been previously transferred, and thus overlaps the yellow image. Third and fourth colors, for example, cyan and black, are developed in the same manner while each developing unit repeats access and retreat movements with respect to the photoreceptive drum 100.
FIG. 3 shows a structure of a conventional developing apparatus of the electrophotographic printer. Here, although a description is limited to the yellow color developing unit 113-Y, it can be equally applied to the other color developing units. The yellow color developing unit 113-Y includes a housing 121 containing toner 120, a developing roller 122 having the toner 120 adhering to the surface thereof and developing an electrostatic latent image of the yellow color formed on the photoreceptive drum 100 with the toner 120, and a supply roller 124 cleaning a surface of the developing roller 122 and supplying the toner 120 to the developing roller 122. A cam 125 and a shock prevention plate 127 are disposed on a rear side of the housing 121 to control the housing 121 to access toward the photoreceptive drum 100. The developing apparatus includes a doctor blade 128 to allow the toner 120 to adhere to the surface of the developing roller 122 with a uniform thickness.
The yellow color developing unit 113-Y accesses or retreats with respect to the photoreceptive drum 100 by an operation of the cam 125. In doing so, during one turn of the photoreceptive drum 100, a developing roller of a color to be developed contacts the surface of the photoreceptive drum 100 while developing rollers of the other colors are maintained to be separated therefrom. The yellow color developing unit 113-Y retreats from the photoreceptive drum 100 by a rotation of the cam 125 after the development of the yellow color is completed. Then, another color developing unit of a subsequent color accesses the photoreceptive drum 100.
Consequently, all colors are completely developed through the above development processes, and a desired color image is finally formed on the transfer belt 114. Here, in order to transfer the respective color images onto the transfer belt 114 to overlap one another, the photoreceptive drum 100 and the transfer belt 114 need to be matched in rotation period, and the photoreceptive drum 100 and the transfer belt 114 rotate one time for each color. Then, the desired color image completely formed through the above development processes is transferred to the sheet of paper supplied between the transfer belt 114 and the second transfer roller 114b. While passing through the fusing unit 115, the paper is heated and pressed so that the color image is completely fixed to the paper. The paper is exhausted along the transfer path 119.
However, since the photoreceptive drum 100 is cylindrical, when the developing units 113-Y, 113-M, 113-C, and 113-K for yellow, magenta, cyan, and black colors, respectively contact the photoreceptive drum 100 in the development processes, contact angles between the photoreceptive drum 100 and the respective developing rollers 122 are different. Thus, since a direction of a force received from a contact portion of each developing unit against the photoreceptive drum 100 is different from that of other developing units, a constant pressure and position is not maintained during performing the development processes. Also, the developing roller 122 cannot stably contact the photoreceptive drum 100 in response to a linear movement of the developing unit 113 by the cam 125. Further, apparatuses for driving the cam 125 are additionally installed in the conventional developing apparatus. In a case of the printer shown in FIG. 1, since the paper transfer path 119 is complicatedly located in a rear side of the developing unit 113, it is difficult to secure a space for the additional apparatuses in the conventional electrophotographic printer.
To solve the above and other problems, the present invention provides a developing apparatus of an electrophotographic printer in which a contact force of the developing roller for each color is uniformly applied with respect to the photoreceptive drum, and the developing roller is coupled to the photoreceptive drum such that a shock can be minimized.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
According to one aspect of the present invention, a developing apparatus of an electrophotographic printer for developing an image on a photoreceptive drum comprises a disc rotating independently of the photoreceptive drum, disposed on at least one of both sides of the photoreceptive drum, and having at least one cam groove formed on an outer circumferential surface thereof, a developing unit including a housing containing a developer and a developing roller supported by both side walls of the housing and having a shaft contacting the outer circumferential surface of the disc, and an elastic member provided at a rear surface of the housing and pressing the developing unit toward the photoreceptive drum.
According to another aspect of the present invention, the disc has a diameter greater than that of the photoreceptive drum. When the shaft is disposed in the cam groove, the developing roller contacts the photoreceptive drum. The disc is installed to be coaxial with the photoreceptive drum. A bearing is provided around the shaft so that the bearing is disposed to rotate between the shaft and the outer circumferential surface of the disc. The developing apparatus further comprises a power transfer unit using a motor or a gear to drive the disc. Two or more cam grooves are provided at an identical interval along the outer circumferential surface of the disc. The above developing apparatus further comprises a supply roller installed adjacent to the developing roller to supply the developer to the developing roller.